Micro-electromechanical system technology has spurred the development of different types of micro-pumps to transport liquids over a wide range of flow rates and pressures. Micro-pumps basically fall into one of two groups: membrane displacement pumps and field-induced flow pumps. In membrane displacement pumps, the deflection of a membrane provides the pressure for pumping liquid. Conversely, field-induced pumps operate via electroosmotic flow (“EOF”). Field-induced pumps have an important advantage in that this type of pump does not require moving parts that complicate fabrication and sealing.
In an electrokinetic (“EK”) pump, also known as an “electroosmotic pump,” precise flow control is driven by an applied electric field and achieved simply with current or a voltage-controlled circuit. In such pumps, the potential is applied at the ends of the capillary and viscous drag creates EOF. Fluid migrates towards positive electrode in positive pumps and towards the negative electrode in negative pumps.
By design, EK pumps have low stall pressure. Generally, prior art open channel and capillary EK pumps are not used in systems with high-pressure loads. Unlike mechanical pumps, electroosmotic flow does not produce a pressure field. In order to achieve high pressure, the pump channel must be very small to produce hydrodynamic resistance. This is opposite of a mechanical pump where hydrodynamic resistance reduces the flow and build up of high pressure. However, in the EK pump, the hydrodynamic resistance effects a reduction in the fluid flow in the direction opposite that of electroosmotic flow; this results in a net build-up of pressure at the channel outlet. Hence, pump channels need to be as small as possible, but not smaller than the double layer thickness of the electrolyte. Moreover, because a single small pump channel limits the maximum flow, a large bundle of micro-channels is needed.
Conventional EK pumps, both open channel and capillary, can fail as a result of bubble generation. For example, in an open channel system, when the applied electrode potential exceeds a certain threshold voltage, bubbles are generated as a result of electrolysis and other electrode reactions. Ions are produced that contaminate the sample and block the micro-channels. To eliminate blockage, a bubble releasing device must be used downstream of the pump or bubbles must be allowed to escape via open reservoirs. If an open reservoir is used, the housing must be capable of electric field penetration. If the EK pump performance can be improved, as measured by the “EK factor”, defined as the maximum pressure achieved for a given applied voltage (“psi/V”) in a closed channel, then fewer bubbles will be generated without the need for such bubble releasing devices.
Attempts have been made to reduce the electrode reaction and low-pressure disadvantages of the EK pump. One approach has been to pack capillaries or channels with small particles containing high surface charge that still allows for electro-osmotic flow. However, packing the column is very difficult, particularly when particles less than ca. 1 micron diameter are used. With such particles, extremely high pressures are required, which often cannot be obtained due to the limitations in mechanical strength of the device. In addition, long times are required to pack such columns, which can add substantially to the cost of the device.
Hence, monoliths have been recently been considered for use as components in high-pressure EK pumps. Monoliths have been synthesized using organic and inorganic materials and combinations thereof. While organic materials are chemically stable under alkaline and acidic conditions, these materials shrink and swell when subjected to different solvents. In addition, they do not have the mechanical strength of inorganic silica materials. On the other hand, silica-based monoliths suffer from shrinkage during synthesis and large voids between the monolith and the capillary wall/channel wall can result.
EK pumps using monolithic compositions have been suggested. See e.g., U.S. Pat. No. 6,290,909, col. 3 at line 36 where polymer monoliths are selected to resist pressure-driven flow and allow electroosmotically-driven flow. These monolithic compositions, however, lack rigidity and small domain size needed for high pressures. They also contain significant meso-porosity which would limit their performance as EK pumps due to the double-layer overlap which occurs in the mesopores.
Further, hybrid inorganic-organic monolith materials have been investigated as chromatographic stationary phases to improve adhesion of the monolith to the capillary wall and increase resistance to shrinkage. See e.g., WO 2004/105910. However, these materials contain significant meso-porosity which would limit their performance as EK pumps due to the double-layer overlap which occurs in the mesopores.
A need exists, therefore, for an EK monolithic pump that can produce higher pressures over a wide range of pH while maintaining a strong and stable morphology over a long lifetime.